The semiconductor device fabrication process uses plasma processing at different stages of fabrication to make a semiconductor device such as a microprocessor, a memory chip, or another integrated circuit or device. Plasma processing involves energizing a gas mixture by imparting energy to the gas molecules by the introduction of RF (radio frequency) energy into the gas mixture. This gas mixture is typically contained in a vacuum chamber, also called a plasma chamber, and the RF energy is introduced through electrodes or other means in the chamber. In a typical plasma process, the RF generator generates power at the desired RF frequency and power, and this power is transmitted through the RF cables and networks to the plasma chamber.
To provide efficient transfer of power from the RF generator to the plasma chamber, an RF matching network is positioned between the RF generator and the plasma chamber. The purpose of the RF matching network is to transform the plasma impedance to a value suitable for the RF generator. In many cases, particularly in the semiconductor fabrication processes, the RF power is transmitted through 50 Ohm coaxial cables and the system impedance (output impedance) of the RF generators is also 50 Ohm. On the other hand, the impedance of the plasma, driven by the RF power, varies. The impedance on the input side of the RF matching network must be transformed to non-reactive 50 Ohm (i.e., 50+j0) for maximum power transmission. RF matching network perform this task of continuously transforming the plasma impedance to 50 Ohm for the RF generator.
A typical RF matching network is composed of variable capacitors and a microprocessor-based control circuit to control the capacitors. The value and size of the variable capacitors are influenced by the power handling capability, frequency of operation, and impedance range of the plasma chamber. The predominant variable capacitor in use in RF matching networks is the vacuum variable capacitor (VVC). The VVC is an electromechanical device, consisting of two concentric metallic rings that move in relation to each other to change the capacitance. In complex semiconductor processes, where the impedance changes are very rapid, the rapid and frequent movements put stresses on the VVC leading to their failures. VVC-based RF matching networks are one of the last electromechanical components in the semiconductor fabrication process.
As semiconductor devices shrink in size and become more complex, however, the feature geometries become very small. As a result, the processing time to fabricate these features becomes small, typically in the 5-6 second range. Current RF matching networks take 1-2 seconds to tune the process and this results in unstable process parameters for a significant portion of the process time. Electronically variable capacitor (EVC) technology (see, e.g., U.S. Pat. No. 7,251,121, incorporated herein by reference) enables a reduction in this semiconductor processing tune time from 1-2 seconds to less than 500 seconds. EVC-based matching networks are a type of solid state matching networks. Their decreased tune time greatly increases the available stable processing time, thereby improving yield and performance.
While mechanical RF matching networks, such as VVC-based matching networks, cannot respond to rapid impedance changes, solid state RF matching networks, such as EVC-based matching networks, are able to respond to load impedance changes in a micro-second time frame. While this rapid response time is advantageous in applications such as when matching the load impedance of a semiconductor plasma, it can sometimes result in making the plasma instabilities worse by rapidly responding to impedance changes which may be so timed that the changes in the solid-state matching network form a positive feedback. To prevent further destabilizing a plasma load, but still providing a rapid response to match the plasma load impedance, a new matching algorithm is needed.